World's most sensitive model-independent experiment starts searching for dark matter

Today at DESY, the "light shining through a wall" experiment ALPS II, the most sensitive model-independent experiment in the world to look for especially light particles, of which dark matter may be comprised, begins. According to mathematical calculations, this terrifying type of stuff should be present in the cosmos five times more frequently than regular, visible matter. However, no one has yet been able to distinguish this substance's particles; the ALPS experiment might potentially provide such proof.

A particularly light sort of new elementary particle is being sought after by the 250 meter-long ALPS (Any Light Particle Search) experiment. The multinational research team is looking for these so-called axions or axion-like particles using 24 repurposed superconducting magnets from the HERA accelerator, a strong laser beam, precise interferometry, and incredibly sensitive detectors.

Since these particles are thought to interact with known types of matter only very weakly, they cannot be found in tests utilizing accelerators. Since photons, or light particles, may transition into these enigmatic fundamental particles and back again under a strong magnetic field, ALPS is thus using a completely different technique to identify them.

"For more than 30 years, the concept for an experiment like ALPS has existed. We are now able to achieve ALPS II in an international partnership for the first time thanks to the use of parts and the infrastructure of the previous HERA accelerator in combination with cutting-edge technology, says Beate Heinemann, Director of Particle Physics at DESY.

The Chairman of the Board of Directors of DESY, Helmut Dosch, continues, "DESY has assigned itself the mission of decoding matter in all its many forms. Therefore, ALPS II is a wonderful fit for our study methodology and might help to unlock the mystery of dark matter.

A high-intensity laser beam is sent by the ALPS team down an optical resonator in a vacuum tube that is 120 meters long and encircled by twelve HERA magnets placed in a straight line, causing the beam to be reflected back and forth. A photon may pass through the opaque wall at the end of the line of magnets if it were to transform into an axion in the strong magnetic field.

It would enter a second magnetic track that was nearly identical to the original one after passing through the wall. The axion may then undergo a transformation back into a photon at this point, which would be detected at the conclusion. Here we put up a second optical resonator to boost by a factor of 10,000 the likelihood of an axion converting back into a photon.

If light does penetrate the wall from behind, an axion must have been present. But despite all of our technical gimmicks, adds DESY's Axel Lindner, project coordinator and ALPS partnership spokesperson, "the probability of a photon turning into an axion and back again is very small, like throwing 33 dice and they all coming up the same."

The researchers had to optimize every aspect of the apparatus to ensure that the experiment would function as intended. One photon may be detected every day by the light detector due to its extreme sensitivity. The mirror system for the light has a record-breaking level of accuracy; the space between the mirrors must remain constant in relation to the laser's wavelength to within a fraction of an atomic diameter.

Additionally, the nine-meter-long superconducting magnets create a 5.3 Tesla magnetic field in the vacuum tube, which is more than one million times stronger than the Earth's magnetic field. The magnets were repurposed for the ALPS project from the HERA accelerator's 6.3-kilometer-long proton ring. In order for the magnets to hold more laser light during the experiment, they had to be straightened on the inside. Additionally, the safety equipment needed to operate the magnets at minus 269 degrees Celsius when superconducting has undergone a comprehensive overhaul.

The theory behind the ALPS experiment was first put out by DESY theorist Andreas Ringwald, whose calculations on expanding the Standard Model served as the theoretical foundation for the experiment. For ALPS, theoretical and experimental physicists collaborated closely, according to Ringwald. As a result, an experiment has been created that has a special chance of finding axions, which we may someday employ to look for high-frequency gravitational waves.

To make it easier to look for "background light" that could mistakenly suggest the presence of axions, the search for axions will first start in an attenuated operating mode. Full sensitivity for the experiment is anticipated in the second part of 2023. In 2024, the mirror system will be improved, and a different light detector can be added at a later date.

In 2024, the researchers aim to publish the initial findings from ALPS. "Even if we don't find any light particles with ALPS," asserts Lindner, "the experiment will shift the exclusion limits for ultra-light particles by a factor of 1000."

In total, some 30 scientists have joined the global ALPS cooperation. They are affiliated with seven academic centers, including DESY, the Max Planck Institute for Gravitational Physics (Albert Einstein Institute), the Institute for Gravitational Physics at Leibniz University in Hanover, Cardiff University (UK), the University of Florida in Gainesville, Florida, the Johannes Gutenberg University in Mainz, the University of Hamburg, and the University of Southern Denmark in Odense.

Planning has already begun for the period after the present quest for axions. For instance, scientists aim to employ ALPS to determine if a magnetic field, as predicted by Euler and Heisenberg decades ago, affects the propagation of light in a vacuum. The experimental set-up will also be used by the researchers to look for high-frequency gravitational waves.

Describe axions.

Theoretical elementary particles called axions. They are a component of a physical process that theorist Roberto Peccei and his collaborator Helen Quinn proposed in 1977 to address an issue involving the strong interaction, one of the four basic forces of nature. Theoretical physicists Steven Weinberg and Frank Wilczek connected a new particle to this Peccei-Quinn process in 1978.

Wilczek gave the particle the name "axion" after a detergent since it would "clean up" the theory. Axions or axion-like particles are predicted to exist by a variety of extensions to the Standard Model of particle physics. If they are real, they would provide answers to a number of issues that are now troubling physicists, including potential candidates for the components of dark matter. Current estimations predict that the cosmos should contain around five times as much dark matter as regular matter.